Alloy steels for use at low temperatures

ABSTRACT

An alloy steel for use in a low-temperature region consisting of C in the range from 0.02% to 0.2% Ni in the range from 3% to 8%, Cu in the range from 0.8% to 4%, A1 in the range from 0.005% to 0.3%, Si in the range from 0.05% to 0.15%, Mn in the range of 0.2% to 1.2%, and the balance iron and impurities, of which the transition temperature is lower than -196* C., a charpy impact value is greater than 2.1 Kg.-m. at said temperature and a tensile strength greater than 73 kg./mm.2 at 20* C.

Field of Search ..75/l23, 124, 125; 148/36 United States Patent [151 3,635,770 Sasakl et a]. Jan. 18, 1972 [5 ALLOY STEELS FOR USE AT LOW [56] References Cited T 'l U RE EMPERA S UNITED STATES PATENTS [72] Inventors: Ryoichi Sasaki; Toru Irino, both of 2 012 765 8,1935 l-lltachl-shi, Japan 2:l68:561 8,1939 [73] Assignees: Hitachi, Ltd.; Hitachi Metals, Ltd., Tokyo, 17 1 H1950 Japan 2,797,162 6/1957 2,867,531 1 1959 [21] Appl. No.: 854,265 3,290,128 12/1966 3,365,343 1/1968 Related US. Application Data [63] Continuation-impart of S61. No. 457,167, May 19, i: ;fi 1965 abandmed- Attomey-Craig, Antonelli & Hill [30] Foreign Application Priority Data [57] ABSTRACT a g2 japan "39/28148 An alloy steel for use in a low-temperature region consisting of ar. apan C in the range frOm to in the range f to 8%, Cu in the range from 0.8% to 4%, Al in the range from [52] US. Cl. ..148/36, 75/124, 75/125, 00059, to 03% Si in the range from o 05% to (5%, Mn in 51 I t Cl 75/128 75/128 8,37 the range of 0.2% to 1.2%, and the balance iron and impurill ..C22 ties, of the transition temperature is lo e th n 9 o 1 C., a charpy impact value is greater than 2.1 Kg.-m. at said temperature and a tensile strength greater than 73 kg./mm." at 20 C.

8 Claims, No Drawings ALLOY STEELS FOR USE AT LOW TEMPERATURES The present application is a continuation in part application of Ser. No. 457,167 filed on May 19, 1965, now abandoned.

This invention relates to an alloy steel adapted for use in a low-temperature region and more particularly, to an alloy steel having excellent mechanical properties at low temperatures.

DESCRIPTION OF THE PRIOR ART Recently remarkable development has been made in chemi cal industrial apparatus of the kind adapted for operation in a low-temperature region, such as apparatus for the production, transportation and storage of liquid oxygen and liquid nitrogen and liquified gas separators in petrochemistry. In these chemical apparatus, alloy steels are generally widely employed as a material for apparatus components such as valves. Steelmaterial is generally defective in a low-temperature region in that its toughness is lost in such region and:it tends to develop brittle fracture in an extremely low-temperature region such as -196 C., however. These defects which the steel material shows at low temperatures are believed that the material has poor impact strength at such low temperatures. Further it is said that the steel material must generally have an impact value at least 2.l kg.-m. at a given low temperature when measured by the Charpy impact testing.

To meet the demand as described above, a steel containing 18 percent by weight of chromium and 8 percent by weight of nickel or a steel containing more than 8 percent by weight of nickel has been employed in a temperature region down to l96 C. These materials has considerably excellent toughness in the respective operating temperature regions but have poor hardenability with the result that the cooling velocity during quenching becomes quite slow in case of largesized members and it is difficult to give a sufficient heat treatment effect depth into the deep portion thereof. Therefore this results in a considerable difference between the strength or impact value of the deep portion and that of surface and in the vicinity thereof. Water quenching, oil quenching or the like may be considered as a means of giving a sufficient heat treatment effect depth into the central portion of an article of steel material, but such quenching treatment is objectionable for a large-sized member because quenching cracks are liable to develop. Thus it is difficult in the case of large-sized members to provide a sufficient strength and an impact value to the deep portion thereof.

Further inclusion of a large amount of nickel causes an increased cost of material which results in a considerably high cost of products in case of large-sized members, and thus use of nickel to large-sized members is limited. Therefore, remarkable advantages will be derivable in the manufacture of chemical apparatus of the kind if it is possible to obtain a steel material suitable for use in a low-temperature region as referred to above which has a low content of nickel, has a Charpy impact value of more than 2.1 kg.-m. in its operating temperature region and have satisfactory hardenability.

SUMMARY OF THE INVENTION One of the objects of the present invention is to provide a novel alloy steel for use at low temperatures.

Another object is to provide an alloy steel which has an excellent heat treatment effect and has a Charpy impact value of more than 2.1 kg.-m. in a low temperature such as -196C. or lower.

Still other object of the present invention is to provide an alloy steel for use at low temperatures which is suitable for material of components such as valves of liquifaction apparatus for operation at low temperatures.

The above and other objects, features and advantages of the present invention will become apparent from the following description.

Advantages derivable from the alloy steels of the kind described above are that they can satisfactorily be used to form large-sized articles by virtue of their excellent hardenability, and the desired object can sufficiently be satisfied even with a nickel content less than the prior content, with the result that reduction of product cost can easily be attained.

The invention disclosed herein is not merely based on the fact that copper, molybdenum and chromium act to increase the toughness of steel and thus are effective to reduce the nickel content, but is based on an entirely novel discovery that addition of copper to a nickel steel containing aluminum therein can uniformly and remarkably improve mechanical properties such as tensile strength and Charpy impact value of the steel and further addition of molybdenum and chromium thereto can improve the hardenability of the steel.

Or more precisely, the present invention relates to an alloy steel for use at low temperature which consists essentially of from 0.02 to 0.2 percent by weight carbon, from 3 to 8 percent by weight nickeLfrom 0.8 to 4 percent by weight copper, from 0.005 to 0.3 percent by weight aluminum, from 0.05 to 0. 15 percent by weight silicon, and from 0.2 to 1.2 percent by weight manganese. According to the invention molybdenum and chromium may singly or compositely be added to the alloy steel within a range of a suitable amount so as to improve the mechanical properties.

In the alloy steel according to the invention, increase in the carbon content to a certain extent can increase the strength of the steel. With a carbon content of less than 0.02 percent by weight, the steel can not have a satisfactory strength and its hardenability is degraded. Further with a carbon content of more than 0.2 percent by weight, weldability and toughness of the steel are lowered and its Charpy impact value is also lowered. Therefore the carbon content should be limited within a range of from 0.02 to 0.2 percent by weight, and a most preferred range of the carbon content in order that the alloy steel can exhibit sufficiently satisfactory mechanical properties is from 0.05 to 0. l 3 percent by weight.

Aluminum is added in a range of 0.005 to 0.3 percent by weight as a deoxidizing agent and in order to fine crystal grains and to improve the toughness of the alloy steel. That is, aluminum of such an amount as is ordinary added as a deoxidizing agent is effective to fine crystal grains and to improve the toughness. An aluminum content ofless than 0.005 percent by weight can not attain satisfactory deoxidization and improvement in the toughness, however. Though addition of aluminum of more than 0.3 percent by weight exhibits an excellent effect in the fining of crystal grains, the deoxidizing effect reaches its limit and the toughness is rather degraded. There fore, an appropriate amount of addition of aluminum should be limited within a range of from 0.005 to 0.3 percent by weight, and a range of from 0.03 to 0.l5 percent by weight is more preferred in order that the effect of aluminum addition can most satisfactory be exhibited. It is. usually said that titanium also acts as deoxidizing agent in a steel and fines crystal grains as well as aluminum. According to the investigations of the inventors, however, it has been found that addition of titanium deteriorates the toughness of the steel at low temperatures.

Nickel in the alloy according to the invention exhibits a remarkable effect in the improvement of its tensile strength and forgeability. Also when coexistent with copper, nickel improves the impact value of the steel and effects a remarkable improvement in the low-temperature toughness of the steel. However, the steel can not have the Charpy impact value of more than 2.1 kg.-m. in a low-temperature region even with the addition of the other alloying elements when the amount of nickel is less than 3.5 percent by weight and the steel will have an excessively highmaterial cost when nickel is added in an amount more than 7 percent by weight. Therefore, it is necessary to limit the amount of addition of nickel within a range of from 3.5 to 7 percent by weight.

According to the invention, marked effect can be obtained for a temperature down to l96 C. by the addition of from 3 to 8 percent by weight. The steels having the respective nickel contents have a Charpy impact value of more than 2.1 kg.-m. in the respective operating temperature regions.

The alloy steel composed by these alloying elements may include a suitable amount of silicon, manganese and the like as a desulfurizing or deoxidizing agent. Further as described above, the alloy steel of the invention may include such impurities as nitrogen, phosphorous and sulfur which are ordinary involved in iron and the other alloying elements. These impurities would not adversely affect the steel properties if each impurities is included in amount less than 0.03 percent by weight.

Silicon when added as a deoxidizing agent exhibits an excellent effect. Silicon added in an amount less than 0.05 percent by weight can not effect sufficient deoxidization with the result that steel obtained have not enough impact value for the purpose, while the deoxidization effect is no more improved when silicon is added in amount more than 0.4 percent by weight. Thus it is necessary to limit the amount of silicon to a range of from 0.05 to 0.4 percent by weight, and the amount of silicon should preferably be limited to a range of from 0.1 to 0.3 percent by weight in order to attain the best effect of the addition of silicon.

Just like silicon, manganese is used as a deoxidizing agent and also has a function as a desulfurizing agent. Manganese effects insufficient deoxidization and desulfurization and can not impart satisfactory forgeability to the alloy steel when it is added in an amount less than 0.2 percent by weight. On the other hand, the effect of deoxidization and desulfurization by manganese is saturated and shows no more improvement when manganese in excess of 1.2 percent by weight would rather lower the toughness of the alloy steel. Therefore, manganese should be added in an amount of from 0.2 to 1.2 percent by weight, while addition of from 0.4 to 0.9 percent by weight exhibits the most excellent effect when desulfurization and deoxidization are especially required.

Copper contained in the steel of the invention contributes to improvement in heat treatment effect of the steel.

As a result, the steel has an excellent low-temperature toughness and high strength. Addition of molybdenum and chromium to this material is effective to further improve its low-temperature toughness, and the preferred range and effect of addition of these elements will become apparent from the following description examplifing several embodiments of the invention.

Table 1 shows chemical composition in percent by weight of various alloy steels. In connection with table 1, it is to be un derstood that addition of copper, molybdenum or chromium to nickel-aluminum steels is very usefulfor improvement and increasement in mechanical properties, especially toughness at low temperatures.

TABLE 1.-CHEMICAL COMPOSITION (IN PERCENT BY WEIGHT) Specimen number Si Mn A1 Ni Cu Mo Cr Fe 0.22 0.82 0. 145 1. 08 1.98 0 0 Balance 0.22 0.93 0. 111 1. 11 1.00 0.38 0 Do. 0.23 0.43 0.030 1. 28 1.02 0. 20 0 D0. 0.20 0.73 0.010 1.10 2.98 0.88 0 Do. 0. 14 0.78 0. 0 80 1. 15 1. 01 0 1.00 D0. 0.14 0.91 0.050 1.03 0.99 0 2.08 Do. 0.27 0.62 0.050 1.34 1. 50 0.60 0.30 Do. 0. 26 0.53 0. 070 1.34 0.83 0. 27 0.05 Do. 0. 15 0. 74 0. 007 3. 55 0. 12 0 0 Do. 0. 21 0.50 0.006 8. 72 0. 13 O 0 Do. 0.30 0.81 0.054 3.09 2.00 0.29 0 Do. 0.22 0.61 0.030 3. 71 2.30 0.58 0 Do. 0. 16 0.62 0. 040 3.88 1.85 0 0.85 Do. 0. 21 0.63 0.030 4.42 0.96 0 0.93 D0. 0.17 0.65 0.070 3.75 1.83 0.61 0.58 D0. 0. 23 0. 60 0. 070 3. 65 2. 76 0.6 0. 34 Do. 0. 13 0. 58 0.030 3.95 1.93 0 1. 17 D0. 0.09 0.46 0.01 3.93 1.73 0.6 0.11 Do. 0.21 0.63 0.03 4.42 0.96 0 0.93 Do. 0. 19 0. 36 0. 008 6. 85 2. 03 0 0 D0. 0. 08 0.53 0.012 6. 98 2.82 0 0 D0. 0. 20 0.46 0.054 6. 85 2.08 2.38 0 D0.

Specimens nos. 1 to 22 shown in table 1 are subjected to heat treatment, that is, heated at a temperature of 900 C. and quenched in oil, then tempered at 650C. and subjected to oil cooling to investigate the effect of addition of copper and further addition of molybdenum and chromium either singly or compositcly on the mechanical properties of the steels at room temperatures. The results are shown in table 2. in table 1 and 2, specimens nos. 1 to 10 are shown only to compare with the mechanical properties of alloy steels of the present invention.

TAB LE 2 5 mm. U-notch Tensile Area charpy impact strength Elonreduevalue (kg.-m.) (kg gation tion Specimen mmfi) (percent), (percent), 20 196 number 20 20 C. 20 C. C. C. C.

It will be apparent from table 2 that the specimens nos. 9 to 10 containing copper in an amount of the order of impurity and not including any of molybdenum andchromium have a Charpy impact value at l00 C. of the order of 7 to 9 kg.-m. though they include nickel in a great amount of 9 to 13 percent by weight and thus the effect of improvement in low nickel is not distinct in these specimens. Further these specimens have a tensile strength which is slightly lower than those of other specimens which include a similar amount of nickel.

However, the specimens nos. 5 to 17 which contain either of both of molybdenum and chromium in addition to copper have a Charpy impact value at 196 C. of far more than 2.1 kg.-m. when they contain nickel in a relatively small amount of 3.5 to 4.5 percent by weight, and when they are subjected to rapid cooling as in the heat treatment. Mechanical properties of the specimens nos. 5 to 17 at an atmospheric temperature of 20 C. are similar to those of the specimens nos. 1 to 4, and there is seen no marked influence by the single or composite addition of molybdenum and chromium.

It is found that those specimens to which either or both of molybdenum and chromium are added and which contain nickel in an amount of 3.5 to 4.5 percent by weight have a Charpy impact value of more than 2.1 kg.-m. in a low-temperature region of 196 C. even when slowly cooled in the quenching treatment and thus have improved hardenability. Therefore it has been made clear that the steel having the alloy composition satisfies the Charpy impact value of 2.1 kg.- m. which is required as a material for a chemical apparatus component such as a valve for operation in a low-temperature region and sufficiently withstand brittle fracture in such lowtemperature region. But the amount of nickel added is necessary to be at least 3 percent by weight to obtain an improved Charpy impact value at low temperatures.

As will be understood from the above description, the alloy steel of the invention for use at low temperatures has improved hardenabi 'ty compared with prior nickel-aluminum steel and has an mpact value required for securing safety against brittle fracture of steel material that may develop in a low-temperature region. These excellent properties for the present invention are attained by adding chromium, molybdenum and copper within the above-mentioned range.

in the present invention, addition of copper to a nickel steel can exhibit a remarkable effect in the improvement of toughness. Addition of copper in an amount less than 0.8 percent by weight can not give sufficient toughness, and addition of copper in an amount in excess of 4 percent by weight the effect of copper addition reaches its limit and forgeability and weldability are rather degraded. Therefore copper is added in an amount of from 0.8 to 4 percent by weight, and copper in an amount of from L5 to 2.5 percent by weight should be contained in the steel in order that the steel can exhibit most excellent properties by the addition of copper therein.

Addition of molybdenum is effective to further increase the low-temperature toughness of the alloy steel and imparts the effect of heat treatment deeply into the interior portion of the thick-walled articles, which can have increased toughness. However, the effect of molybdenum is saturated even with addition in an amount more than 0.8 percent by weight and this is also disadvantageous in respect of economy. The amount of molybdenum should be limited within a range of 0.25 to 0.8 percent by weight in order that the effect of molybdenum can optimumly be exhibited.

The effect chromium is similar to molybdenum in that chromium also imparts the effect of the heat treatment deeply into the intercorpotion of the thickwalled article. Addition of chromium is further effective in that satisfactory toughness and high-tensile strength are thereby imparted irrespective of a slow or quick rate cooling. For the purpose, chromium must be added in an amount more than 0.2 percent by weight. The maximum value of chromium addition is set at 2.5 percent by weight because the effect of chromium addition is saturated at this value and any appreciable improvement in the mechanical properties and low-temperature toughness can not be expected by addition of a further greater amount. Chromium exhibits its best effect when added in an amount of from 0.5 to 1.5 percent by weight and steel material having excellent lowtemperature toughness can be provided by the addition of chromium falling within the above range. When both of molybdenum and chromium are simultaneously added to the steel, it is necessary that the content of each element falls within a range of from 0.5 to 2.0 percent by weight. Molybdenum and chromium added within this range can greatly improve the hardcnability and low-temperature toughness.

What we claim is:

1. An alloy steel for use in a low-temperature region consisting essentially of C. in the range from 0.02 to 0.2 percent, Ni in the range from 3 to 7 percent, Cu in the range from 0.8 to 4 percent. Al in the range from 0.005 to 0.3 percent, Si in the range from 0.05 to 0.15 percent, Mn in the range from 0.2 to L2 percent, and the balance substantially iron and accompanying impurities, said alloy having been heat treated and tempered to exhibit a transition temperature lower than 96 C., a Charpy impact value greater than: 2.l kg.-m. at l96 C. and a tensile strength greater than 73 kg./mm. at 20 C.

2. An alloy steel according to claim 1, wherein the steel further contains Cr in the range from 0.2 to 2.5 percent.

3. An alloy steel according to claim 1, wherein the steel further contains M0 in the range from 0.25 to 0.8 percent.

4. The alloy steel of claim 1, wherein nickel is present in an amount from 3.5 to 7 percent.

5. An alloy steel for use in a low-temperature region consisting essentially of C. in the range from 0.02 to 0.2 percent, Ni in the range from 3 to 7 percent, Cu in the range from 0.8 to 4 percent, Al in the range from 0.005 to 0.3 percent, Cr in the range from 0.2 to 2.5 percent, M0 in the range from 0.25 to 0.8 percent, Si in the range from 0.05 to 0.15 percent, Mn in the range from 0.2 to L2 percent, and the balance substantially iron and accompanying impurities, said alloy having been heat treated and tempered to exhibit a transition temperature lower than -l96 C., a Charpy impact value greater than 2.1 kg.-m. at l96 C. and a tensile strength greater than 73 kg./mm. at 20 C.

6. An alloy steel according to claim 5, in which a total content of Cr and M0 is in a range from 0.5 to 2.0 ercent.

7. The alloy steel of claim 6, wherein the a oying elements are present in an amount from 6.54 to 1.202 percent.

8. The alloy steel of claim 6, wherein the Cu/Ni ratio ranges from 0.217 to 0.756. 

2. An alloy steel according to claim 1, wherein the steel further contains Cr in the range from 0.2 to 2.5 percent.
 3. An alloy steel according to claim 1, wherein the steel further contains Mo in the range from 0.25 to 0.8 percent.
 4. The alloy steel of claim 1, wherein nickel is present in an amount from 3.5 to 7 percent.
 5. An alloy steel for use in a low-temperature region consisting essentially of C. in the range from 0.02 to 0.2 percent, Ni in the range from 3 to 7 percent, Cu in the range from 0.8 to 4 percent, A1 In the range from 0.005 to 0.3 percent, Cr in the range from 0.2 to 2.5 percent, Mo in the range from 0.25 to 0.8 percent, Si in the range from 0.05 to 0.15 percent, Mn in the range from 0.2 to 1.2 percent, and the balance substantially iron and accompanying impurities, said alloy having been heat treated and tempered to exhibit a transition temperature lower than -196* C., a Charpy impact value greater than 2.1 kg.-m. at -196* C. and a tensile strength greater than 73 kg./mm.2 at 20* C.
 6. An alloy steel according to claim 5, in which a total content of Cr and Mo is in a range from 0.5 to 2.0 percent.
 7. The alloy steel of claim 6, wherein the alloying elements are present in an amount from 6.54 to 12.02 percent.
 8. The alloy steel of claim 6, wherein the Cu/Ni ratio ranges from 0.217 to 0.756. 